Coherent techniques, thanks to their high wavelength selectivity and high sensitivity, are considered to be the most promising solution for detecting also ultra-dense wavelength division multiplexed optical signals, use of which is proposed for next generation access and distribution optical networks. In such an application, coherent receivers will have to be made available to the users, and therefore it is essential that they have a limited cost, typical of the products of consumer electronics. Yet, the currently existing coherent receivers, which have been studied for the transport network, employ sophisticated and hence expensive optical and electronic components, which are therefore incompatible with the requirements of a wide diffusion. Other applications that will take advantage of cheap coherent receivers will be the optical front haul/backhaul of cellular networks (i.e. the optical connections of base stations of cellular networks, e.g. according the so-called long term evolution, LTE, or the Common Public Radio Interface, CPRI), the metropolitan networks or the data centres, where a high number of terminals are envisaged.
Coherent receivers that can employ optical components of common use, such as distributed feedback (DFB) lasers, and an analogue signal processing, and hence can be manufactured at low cost, are already known.
An example is disclosed in the paper “ASK Multiport Optical Homodyne Receivers”, by L. G. Kazovsky et al., Journal of Lightwave Technology, Vol. LT-5, No. 6, pages 770-790, June 1989, on which the preamble of claim 1 is based. In this known receiver, the beat between the received signal, which is an amplitude-modulated signal, and the signal from the local oscillator is carried out by means of a multiport optical coupler, e.g. a coupler with three inputs and three outputs, which receives the two signals of which the beat is to be created (i.e., the received signal and the signal from the local oscillator) at two inputs, whereas the third input is not utilised. Thus, three signals, which are each proportional to the optical beat between the received signal and the signal from the local oscillator and are phase shifted by a phase shift which is different for each of the three outputs (0, +120°, −120°, in the ideal case), are present on the coupler outputs. The three signals are independently detected by respective photodetectors, which provide three analogue signals. Such detected signals are then low-pass filtered, squared and combined into a single signal by means of an adder. The single signal is then subjected to a further low-pass filtering.
A second example is disclosed in U.S. Pat. No. 4,732,447, which discloses the application of the receiver also to phase-modulated signal.
Experiments carried out by the Applicant have demonstrated that a receiver of this kind is capable of operating also in ultra-dense wavelength division multiplexing passive optical networks.
In both cases discussed above, the coherent receiver can correctly operate only if the states of polarisation of both the received signal and the signal from the local oscillator coincide. However, the state of polarisation of the signal from the local oscillator is fixed, whereas that of the received signal changes in random manner, since the monomode optical fibres used as transmission lines have birefringence characteristics variable with distance and time. In general, therefore, only a fraction of the field undergoes conversion and a fading, even total, of the signal can occur.
Yet, the prior art receivers discussed above are polarisation sensitive, and hence it is necessary to adopt in them one of the techniques currently employed or proposed for obtaining the independence from the state of polarisation of the received signal in coherent systems. All such solutions entail a considerable increase in the complexity and hence in the cost, thereby making the receivers incompatible with the requirements of large scale diffusion.
In particular, polarisation diversity (see e.g. U.S. Pat. No. 7,555,227) requires duplicating the detection chain for the two orthogonal states of polarisation. This technique is currently used in transport networks using polarisation division multiplexing, in which case the increase in the complexity and hence in the cost is compensated by the increase in the capacity afforded by polarisation division multiplexing. Yet, in the case of the access networks where such a multiplexing is not envisaged, duplicating the receiver structure only results in doubling the manufacturing costs and the energy consumption.
Among the other known techniques, polarisation modulation at the transmitting side (see U.S. Pat. No. 5,127,066) and automatic polarisation alignment (see U.S. Pat. No. 7,307,722) entail using additional components (e.g. polarisation modulators), which are expensive per se and moreover cause degradation of the performance.
Siuzdak J. et al., “BER Evaluation for Phase and Polarization Diversity Optical Homodyne Receivers Using Noncoherent ASK and DPSK Demodulation”, Journal of Lightwave Technology, Vol. 7, No. 4, April 1989, pages 584-599, discloses two other alternative strategies: a phase diversity homodyne receiver equipped with a polarisation control for making the states of polarisation of the received signal and the signal from the local oscillator coincide, and a polarisation and phase diversity homodyne receiver with polarising beam splitters on the paths of both the received signal and the signal from the local oscillator.
DE 38 21 438 A1 discloses a polarisation-independent heterodyne receiver, where the received signal and the signal from the local oscillator are combined by means of a network of 2×2 couplers and detected by three photodiodes. Yet, this scheme does not provide for phase diversity and this entails that the receiver has necessarily to be employed in heterodyne mode. This has considerable drawbacks: first, heterodyne detection requires use of components with wider bands and reduces receiver sensitivity; moreover, it cannot be used in case of modulation formats according to which the signal is in-phase and quadrature (I/Q) modulated; lastly it requires a very precise control of the frequency difference between the signal from the local oscillator and the received signal. Moreover, the analogue processing of the electrical signals resulting from the photoelectric conversion is rather complex. Furthermore, use of a heterodyne receiver limits the possibility of use in a wavelength division multiplexing (WDM) system with high channel density.